The invention relates to plastic scintillator-based gamma-ray detectors. In particular, the invention relates to surface finishes applied to, and the shape of, plastic scintillation bodies in gamma-ray detectors.
In most commonly-used inorganic (crystal) scintillation counters, the volume of the scintillation body is typically less than 1 liter. For the largest of these detectors, a large diameter photomultiplier tube (e.g. around 8 cm diameter) is used to maximise the collection of photons from gamma-ray interaction events in the scintillation body. The surface finish of such scintillation bodies is typically a finely roughened surface with the crystal packed either in a highly reflective powder (eg MgO), or diffusely reflecting white paper or plastic.
Different surface finishes are generally required for plastic scintillator-based gamma-ray detectors. Plastic scintillators are frequently used in applications where large sensitive areas are required, e.g., on the order 1 m2. The scintillation bodies are often not more than a few cm thick. Large area and relatively flat scintillation bodies such as this are sometimes referred to as scintillator planks. For a generally rectangular plank, the sides having the largest surface may be referred to as faces, the sides having the next largest surface area may be referred to as edges, and sides with the smallest surface area may be referred to as ends.
Most large area plastic scintillation counters are cast between two float-glass plates. This provides the faces of the scintillator plank with a near-optical quality finish. As a consequence, total internal reflection (TIR) readily occurs at these surfaces. The edges of the scintillator planks are typically machined, e.g. using a diamond milling tool, and then polished with a view to providing a similar optical quality surface. Because TIR has a better reflectivity than any other known surface finish, the use of reflecting powders or other diffusively scattering material of the kind used with inorganic scintillation bodies is avoided for large plastic scintillators.
FIG. 1A schematically shows a cross-section view of a gamma-ray detector 2 based on a conventionally packaged plastic scintillation body 4. The scintillation body 4 is made of NE-102 plastic scintillator and has dimensions 150 cm×25 cm×4 cm. The view of FIG. 1A is taken in a plane through the centre of the plastic scintillation body 4 and parallel to its faces. One end surface of the scintillation body 4 is coupled to a photo-multiplier tube (PMT) 8 for detecting photons generated in gamma-ray interactions in the scintillator.
The other surfaces of the scintillation body 4 (i.e. its two faces, two sides, and the end not coupled to the PMT) are wrapped in a layer of aluminium foil 10. Conventional aluminium foil of the kind used in cooking is typically used. The aluminium foil layer is “crinkled” so that it is not in direct contact with the scintillation body over most of its surface. This helps to reduce the effect of the aluminium foil on TIR occurring at the scintillation body surface. The perceived benefit of the aluminium foil is in helping to reflect photons that have exited the scintillation body (i.e. photons that did not undergo TIR) back into the scintillation body. The aluminium foil 10 is wrapped in a layer of a black vinyl 12 for protection and to help to ensure the assembly is light-tight so as to prevent photons not associated with gamma-ray interactions within the scintillator from reaching the PMT.
FIG. 1B is a curve showing the modelled light collection efficiency (LCE) for the detector 2 shown in FIG. 1A. The LCE is shown as a function of distance D from one end of the scintillation body (the end coupled to the PMT) to the gamma-ray interaction site. The LCE is the fraction photons generated in a typical gamma-ray interaction which subsequently reach the sensitive area of the PMT.
As can be seen from FIG. 2, the maximum LCE is around 45%. This is for interactions occurring towards the end of the scintillation body 4 which is nearest to the PMT 8. For gamma-ray interactions occurring at the other end of the scintillation body, the LCE is just over 30%.
In a scintillator-based gamma-ray detector such as shown in FIG. 1A, the energy deposited by gamma-rays (i.e., the energy loss spectrum) is determined from the light energy recorded at the PMT as a result of each gamma-ray interaction. An estimate of the incident gamma-ray spectrum is then derived from the energy loss spectrum by taking account of how the detector responds to incident gamma-rays. It is therefore important to measure the energy loss spectrum (i.e., the energy deposited by gamma-rays in the scintillation body) as accurately as possible if the best possible estimates of the incident gamma-ray spectrum are to be obtained. In an ideal detector, a given deposition of energy in the scintillation body will lead to a given signal from the PMT. Thus a measured output signal can be converted to an estimate of the energy lost in the scintillation body by an incident gamma-ray. Accordingly, the LCE, and its variance with interaction position, plays an important role in determining the energy resolution of a gamma-ray spectrometer based on a scintillator detector. Firstly, a poor LCE leads to poor counting statistics with a corresponding uncertainty in the output signal measured by the PMT. Secondly, in addition to this statistical noise, the variation in LCE with position means that an interaction near to the PMT in FIG. 1A leads to an output signal from the PMT that can be as much as 50% higher than that from an interaction at the other end of the scintillation body. If a linear relationship between PMT output signal and incident gamma-ray energy is assumed, this variance can lead to a 50% uncertainty in an incident gamma-ray photon's energy.
Most applications for large area/volume plastic scintillator detectors are as triggering devices in charged particle detectors. For these applications the significance of the LCE and its variance are frequently not of concern. However, there has been recent interest in using large area plastic scintillator-based detectors as relatively low-resolution gamma-ray spectrometers [1]. In these cases there is a need to improve the LCE and reduce variations in response as much as possible because of the influence of these factors on the energy resolution of such spectrometers.
FIG. 2A schematically shows a cross-section view of another known gamma-ray detector 22. This detector 22 employs a known technique for increasing LCE and reducing its variance [1]. Features of the detector 22 shown in FIG. 2A which are similar to, and will be understood from the corresponding description of, features of the detector 2 shown in FIG. 1A are identified by the same reference numerals. Thus the detector again comprises a 150 cm×25 cm×4 cm NE-102 scintillation body 4 coupled to a PMT 8 for detecting photons generated in gamma-ray interactions in the scintillator. However, in addition to this PMT 8, the detector 22 comprises a second PMT 28. The second PMT 28 is at the opposite end of the scintillation body to the first PMT 8. Thus photons can be detected at both ends of the scintillation body 4. The exposed sides of the scintillation body 4 (i.e. its two faces and two edges) are wrapped in a layer of aluminium foil 30 with an overlying layer of a black vinyl 32 in the same way as described above for the detector 2 in FIG. 1A.
FIG. 2B is a curve showing the modelled LCE as a function of position D measured from the left-hand end of the scintillation body 4 (for the orientation shown in FIG. 2A). FIG. 2B is similar to, and will be understood, from FIG. 1B. However, three curves are shown in FIG. 2B. The curve marked LEFT shows the modelled LCE for the PMT 8 at the left-hand end of the scintillation body 4. The curve marked RIGHT shows the modelled LCE for the PMT 28 at the right-hand end of the scintillation body 4. The curve marked SUM shows the modelled LCE obtained by summing the outputs from both PMTs 8, 28.
The curve marked LEFT is similar to the curve shown in FIG. 1B, but shows a slightly lower LCE. This is because photons that would have been reflected from the end of the scintillator body opposite the PMT 8 in FIG. 1A, are not reflected in the detector 22 of FIG. 2A, but are instead coupled out of the scintillation body to the second PMT 28. Due to the symmetry of the detector the curve marked RIGHT is in effect a mirror image of the curve marked LEFT.
It can be seen from the curve marked SUM in FIG. 2B that by summing the signals from both PMTs 8, 28, the LCE is increased to a relatively uniform level of around 40% over the central 70% or so of the length of the scintillation body. However, more than half the photons are still being lost, and the LCE still varies considerably over the last 15% or so of the length of the scintillation body towards the ends. The LCE is around 25% higher for gamma ray interactions near to an end compared to interactions near the centre. With an assumed linear relationship between PMT output signal and incident gamma-ray energy, this leads to a 25% uncertainty in incident energy.
Accordingly, there is a need for large area plastic scintillator-based detectors having improved LCE and reduced variation in LCE along their length.